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Final Fantasy 7 Rebirth is full of easter eggs and references, but there's one in particular that its devs included despite not knowing its origin.
Some very light spoilers for the game ahead!
When you, Cloud, and co eventually find yourselves in Corel Prison, there's a well you can go down where you can fight various enemies in a tournament-style battle, similar to the Corneo Colosseum Battle in the first game. You don't technically have to visit it, though there is one questline where you can get an item you need to progress. If you do so, or just visit it later, you can fight a mutated dog enemy called Test 0, which does appear in the original game. However, by original, I mean very original - it could only spawn in the first print of Japanese copies of Final Fantasy 7. It wouldn't even attack you, and it had a very low spawn rate anyway, making it overall a bit of a mystery.